The present invention relates to instrumentation for photoelectrically determining the position of at least one focus-plane of an image within optical equipment comprising optics to image at least one object on at least one spatial frequency filter of an optical image correlator, and further comprising a display of the measured light fluxes passing through the spatial frequency filter.
The state of the art may be ascertained by reference to U.S. Pat. No. 2,254,548 and West German Patents 927,239 of Rudolf Sewig, published May 2, 1955; 961,767 of Herbert Kunz and Utz Lauterbach, published Apr. 11, 1957; 1,103,050 of Fromund Hock, published Mar. 23, 1961; and 1,263,325 of Werner Holle, published Mar. 14, 1968, the disclosures of which are incorporated herein.
The apparatus of the present invention is used to determine the position of the focus-plane of an image projected by optical equipment by means of photoelectric means, but not visual means.
To that end, apparatus comprising electrical correlators are known, in which the partial beams from the appropriate object either are fed simultaneously each to one photoelectric detector or are fed by means of an optical chopper alternatingly to a common photoelectric detector, these detectors being divided into strips for comparing the electrical signals generated in the individual strips by the partial beams, furthermore comprising a display for the comparison results after these detectors.
West German Patent 1,263,325 is based on this state of the art and describes an apparatus in which the outputs from the individual strips of the photoelectric detectors are connected to a single common differential circuit which is followed by a display system so that the sum of the absolute values of the difference signals obtained from the mutually corresponding strips is shown.
The measurements obtained from an apparatus such as disclosed in West German Patent 1,263,325 are noteworthy but fall short of good results for low illumination intensities and low contrasts. For a satisfactory analysis of distance, at least 50 strips (50 image points) are required. The manufacture of such arrays of diodes even with the present state of the art of semiconductor technology is a costly undertaking, so that the use of such an array in optical equipment, for instance in a photographic camera, appears of little advantage.
Again, methods for the focusing of images projected by optical systems are known wherein the bright and dark surfaces of raster-like test objects are applied in rapid sequence to photoelectric transducers the outputs of which on account of the alternating light application are trapezoidal pulses representing a measure of the focus as disclosed in West German Patents 927,239 and 961,767.
These methods as disclosed in West German Patents 927,239 and 961,767 suffer from drawbacks in the sense that only specially prepared test objects may be focused which are made to move quickly mechanically and which furthermore must be located at the site of the object to be measured.
A method is described in U.S. Pat. No. 2,254,548 which tests the focus of imaging optical systems by auto-collimation. The light emanating from a source is guided by the optical system to be adjusted to a photoelectric cell. When two test images coincide with the cell, there is a maximum or minimum of cell output, which is used as the criterion of focus.
The method of U.S. Pat. No. 2,254,548 also suffers from the drawback of requiring a previously prepared and previously installed test object, or several test objects.
Apparatus furthermore is known for the focusing of optical systems, wherein the light beam passing through an optical system is resolved in two parts by optical means and wherein a photoelectrically controlled Foucault edge seeks that position in which it affects in the same ratio the brightness of both parts as disclosed in West German Patent 1,103,050.
However, because of the inhomogeneous brightness distribution, the method of West German Patent 1,103,050 cannot be applied to the image correlation of arbitrary object structures.